1. Field of the Invention
The present invention relates to a magnetic head for performing recording/reproducing of information, for use in a magnetic recording apparatus such a VTR or a magnetic storage device of a computer. In particular, the present invention relates to a laminated magnetic head having a magnetic core including at least one layer of magnetic film sandwiched between substrates, and to a metal-in-gap magnetic head having a magnetic film disposed near a magnetic gap.
2. Description of the Related Art
In recent years, to increase the magnetic recording density, a recording medium having high coecive force has been developed. Thus, it is required to develop a high-performance magnetic head suitable for use in conjunction with such a medium having high coercive force.
To achieve higher recording density, it is desirable to reduce the track width and the gap length of a magnetic head, and also desirable to reduce the coercive force of the magnetic head while maintaining a high saturation flux density and a high permeability.
Recently, in view of the above, a metal-in-gap magnetic head has come to be used practically. In this type of magnetic head, a metallic magnetic film is disposed near a magnetic gap of a magnetic core of ferrite or the like.
To achieve better magnetic characteristics in a high frequency range, there has also been developed a laminated magnetic head having a small track width and having high resistivity thereby reducing eddy-current loss.
In a laminated magnetic head, a magnetic film is deposited on a substrate using a sputtering or evaporation technique, and another substrate is then bonded to the deposited magnetic film thereby producing a magnetic core sandwiched between substrates. In this case, the thickness of the deposited magnetic film defines the track width. Therefore, it is easy to obtain a small track width and thus it is possible to improve the recording density and to prevent the interference with neighboring tracks.
Furthermore, because the magnetic circuit is formed with a thin magnetic film having a thickness of a few .mu.m, the eddy-current loss is reduced and the performance of the magnetic head in a high frequency range is improved.
FIG. 1 illustrates an example of a laminated magnetic head for use in a hard disk storage device of a computer.
The magnetic head 10 shown in FIG. 1 generally comprises: a slider 14 including a pair of air bearing rails 16, 16 disposed parallel to each other; a core portion 18 formed at an end of one of floating rails; and a magnetic core 20, in which the magnetic core 20 is disposed between substrates including the slider 14 and the core portion 18. FIG. 2 is an enlarged view of a portion designated by A in FIG. 1. A magnetic film 20' is sandwiched between substrates 14' and 14" which are also parts of the slider, and a magnetic film 20" is sandwiched between substrates 18' and 18" which are also parts of the core portion 18, wherein the magnetic films 20' and 20" form the magnetic core 20. There is a non-magnetic material forming a magnetic gap 22 between the slider 14 and the core portion 18. There is also adhesive glass 24 between one face of the magnetic core 20 and the substrates 14" and 18" wherein the magnetic core 20 is bonded to the substrates 14" and 18" via the adhesive glass 24.
Such adhesive glass for connecting a magnetic core and a substrate will be referred to as laminate glass hereafter in this invention.
The core portion 18 further includes a coil (not shown) wound around the core portion 18 to form a magnetic head.
In operation of the magnetic head for a hard disk storage device shown in FIG. 1 or 2, the magnetic gap 22 runs floating at a very small height over a magnetic hard disk thereby performing magnetic recording or reproducing.
A magnetic head of this type is produced as follows. First, as shown in FIG. 3, a magnetic film 20 is formed on one side face of a block-shaped substrate 26. Then, laminate glass 24 is coated on the magnetic film 20, and another substrate 26' is placed on it. These substrates are pressed against each other at a high temperature thereby bonding them together. In this bonding process, another laminate glass 24 may also be coated on the substrate 26' as required.
The assembled block is cooled down to room temperature to solidify the laminate glass. The block is then cut and polished so as to separately produce a plurality of sliders and cores having desired shapes.
A slider and a core which have been produced separately are positioned such that both sandwiched magnetic films 20 may make good continuity, and non-magnetic adhesive glass 21 is filled into upper portions of winding holes so as to make bond at the magnetic gap. In this way, a magnetic head 10 such as that shown in FIG. 1 is complete. Hereafter in this invention, this bonding process will be referred to as gap bonding, and the adhesive glass 21 used in the gap boding will be referred to as gap glass.
In the formation of a magnetic film on a substrate, a plurality of magnetic films and insulating films such as SiO.sub.2 may also be deposited alternately to form a magnetic core 33 comprising a plurality of magnetic films 32, 32 and insulating films 34, 34 as shown in FIG. 4. In FIG. 4, reference numeral 24 denotes laminate glass which connects the magnetic core 33 to the substrate 28, and reference numeral 30 denotes a magnetic gap.
In the above-described laminated magnetic head, the magnetic core 20 and the substrate 26 are heated up to a temperature (typically, at 600.degree. C.) at which the laminate glass 24 become melted thereby bonding the magnetic core 20 to the substrate 26 under applied pressure, and then cooled down to room temperature.
Substrates are made of a material such as Zn ferrite, MnO-NiO-based ceramic, or TiO.sub.2 -CaO-based ceramic. The magnetic film is usually made of sendust, amorphous alloys, or microcrystal alloys. However, there is a difference in coefficient of linear thermal expansion between the substrate material and the magnetic film material, which induces strain during the cooling process. In general, the substrate material has a smaller coefficient of linear thermal expansion than the magnetic film material, and therefore a greater compressive stress is induced in the magnetic film than in the substrate, and thus strain occurs.
Such strain often brings about a reduction in permeability and an increase in coercive force of the magnetic core. This is a very serious problem with a magnetic head.
For example, FIG. 5 illustrates characteristics of a magnetic head fabricated by: depositing a film of an alloy of Fe.sub.79.6 Ta.sub.10.0 C.sub.10.4 on a substrate of Zn ferrite; performing heat treatment; bonding the same kind of substrate to it via laminate glass (at 700.degree. C.); wherein magnetization curves of the magnetic film measured before and after sandwiched are both shown in the figure. FIG. 6 illustrates similar magnetization curves for the case where a MnO-Ni-based alloy and an Fe.sub.65.2 Al.sub.10.0 Ta.sub.11.2 C.sub.13.6 alloy are employed as the substrate and magnetic film materials, respectively.
In both combinations of a substrate and magnetic film shown in FIGS. 5 and 6, before the magnetic film is sandwiched between substrates, the magnetic film has small coercive force which is suitable for use as a magnetic head. However, after the magnetic film has been sandwiched between substrates, a significant increase in coercive force is observed in both cases.
Even if the magnetic film is not sandwiched between substrates as in metal-in-gap heads, such degradation can occur as long as the magnetic film is deposited on a ceramic substrate or the like.
To avoid the introduction of strain, there have been research and development efforts to obtain substrate and magnetic film materials having coefficients of linear thermal expansion similar to each other.
Thus, it is now very common to use a substrate and a micro-crystal soft magnetic alloy each having an average coefficient of linear thermal expansion as close to each other as possible in the range 110.times.10.sup.-7 to 120.times.10.sup.-7 /.degree.C.
However, in addition to the low strain, the magnetic film also has to meet other requirements such as high resistance to corrosion, high saturation flux density, etc., and thus it is difficult to avoid the reduction in the permeability due to the strain without degradation in any other characteristics. Therefore, it is impossible to obtain a magnetic head satisfying all requirements.
During a heating-up process for heat treatment, a difference in thermal expansion between the substrate and the magnetic film also occurs due to the difference in coefficient of linear thermal expansion. However, the strain is introduced during the cooling process after the heat heating process. This is because of the fact that not only the substrates and the magnetic film but also laminate glass disposed between these materials are softened during the heating process, and thus the softened laminate glass absorbs the difference in thermal expansion between the magnetic film and the substrate. This means that there can be only little strain at a high temperature. In contrast, during the cooling process, in particular at temperatures lower than about 600.degree. C., almost all laminate glass becomes solidified, and therefore the laminate glass can no longer absorb the difference in contraction between the magnetic film and the substrate. Thus, the strain is introduced. The strain is introduced not only at the bonding interface between the substrate and the magnetic film via the laminate glass, but also at the interface between the magnetic film and the substrate on which the magnetic film is deposited. This means that proper selection of coefficients of linear thermal expansion is important not only for laminated magnetic heads but also for other types of magnetic heads such as metal-in-gap magnetic heads, thin film magnetic heads, etc.
In the production process of the above-described laminated magnetic head, after a magnetic core has been bonded between substrates via laminate glass and additional several steps have been carried out, the gap bonding process is carried out to connect magnetic cores together so that a magnetic gap is formed. In the production process, a magnetic head is heated during the gap bonding process so as to melt the gap glass forming the magnetic gap. However, there is some possibility that this heating process causes the laminate glass, which bonds the magnetic core to the substrate, to be melted again.
If the laminate glass which has been solidified once is melted or softened again, the bonding position of the magnetic core can be shifted. This can cause a significant failure. In particular, the softening of the laminate glass degrades the accuracy of the magnetic gap length, which results in a fatal failure of the magnetic head.
One known technique to prevent the above problem is to employ crystallized glass or lead glass as the laminate glass and employ glass having a low or middle melting point as the gap glass. The use of low melting point glass for the gap bonding allows the gap bonding to be performed at a temperature lower than the melting point of the laminate glass so that the laminate glass is not melted during the gap bonding.
In general, soft magnetic alloys for use as the magnetic film include fine crystalline grains, and thus if they are exposed to a high temperature, the crystalline grains grow. This results in degradation in soft magnetic characteristics. Therefore, it is not desirable to expose soft magnetic alloys to a high temperature during the production process. In particular, if these soft magnetic alloys are annealed at about 700.degree. C., a great reduction in resistivity (down to 20-50 .mu..OMEGA.) occurs. As a result, eddy-current loss occurs. This causes a reduction in permeability particularly in a high frequency range, and thus a reduction in reproducing efficiency occurs.
To avoid the above problems with the laminated magnetic head, the thickness of each layer of a multilayer magnetic film is reduced and the number of layers is increased.
However, this technique requires complex production processes. Besides, since the volume ratio of insulating layers in the magnetic core increases, the total saturation flux density decreases.
Furthermore, conventional fine crystalline soft magnetic alloys are generally not good in thermal stability.
Therefore, when a magnetic core is bonded to a substrate, the bonding process should not be carried out at such a high temperature that causes degradation in soft magnetic characteristics of the magnetic film. This means that the laminate glass used for the bonding should not have a very high melting point. Usually, crystallized glass or lead glass is employed as the laminate glass so that the laminating bonding can be performed at about 600.degree. C.
On the other hand, glass having a low melting point is poor in resistance to corrosion. Therefore, as for gap glass, such glass having a melting point in the range of 500.degree. C. to 550.degree. C., especially close to 550.degree. C., is used to obtain as good resistance to environment as possible.
However, the sag point of the above-described laminate glass is about 560.degree. C. This means that the sag point of the laminate glass is only 10.degree. C. higher than the melting point of the gap glass. As a result, the position shift due to the softening of the laminate glass during the gap bonding process still occurs in this technique, and thus production reliability is not good. In the crystallized glass, when it is heated and then cooled, precipitation of crystallized glass occurs whereby its melting point increases. This property is desirable for the application of laminate glass. However, because both crystalline and non-crystalline portions exist in a mixture fashion, the external stress can introduce deformation at the bonding portion even at a temperature lower than the melting point.
Furthermore, conventional fine crystalline soft magnetic alloys are generally poor in resistance to corrosion or environment. One known technique to improve the resistance to corrosion is to add some elements (for example, Cr, Ru, Rh, Al, etc.) to a soft magnetic alloy. However, the addition of these elements causes the soft magnetic alloy to have a positive large saturation magnetostriction constant. Besides, the addition of elements also results in a change in the coefficient of linear thermal expansion of the magnetic film. As a result, it becomes very difficult to achieve optimum characteristics in the combination of the magnetic film and the substrate.
It is an object of the present invention to solve the above problems with a magnetic head produced via a heat-treatment process such as glass bonding. More specifically, it is an object of the present invention to provide a magnetic head having high permeability, low coercive force, and excellent resistance to corrosion, and/or to provide a magnetic head having high production reliability.